1. Field of the Invention
A shaft for collapsible steering apparatus according to the present invention is incorporated in a steering apparatus of automobile and is utilized to transmit a motion of a steering wheel to a steering gear.
2. Related Background Art
In the steering apparatus for an automobile, the mechanism as shown in FIG. 5 is used in order to transmit a motion of the steering wheel to the steering gear. A steering shaft 2 with a steering wheel 1 fixed at the rear end thereof is supported inside a steering column 3 so as to be free only in rotation. This steering column 3 is fixed to the lower surface of an instrument panel 21 through two upper and lower brackets 4, 5. An upper end of an intermediate shaft 7 is coupled through a first universal joint 6 with a portion projecting through a lower end opening of the steering column 3 at the lower end of the steering shaft 2. Further, a lower end of the intermediate shaft 7 is coupled through a second universal joint 8 with an input shaft 9 of a steering gear (not shown). As so arranged, the motion of steering wheel 1 is transmitted through the steering shaft 2 penetrating the steering column 3, the first universal joint 6, the intermediate shaft 7, and the second universal joint 8 to the input shaft 9 of the steering gear, thereby providing the wheels with a steering angle.
The steering mechanism as so arranged generally employs a collapsible arrangement, which allows the total length to decrease upon impact, for the steering column 3, steering shaft 2, and intermediate shaft 7 in order to protect a driver upon collision. Conventionally well known shafts for collapsible steering apparatus employed for that purpose are described for example in Japanese Laid-open Patent Application No. 2-286468 and Japanese Utility Model Publication No. 58-51096. Among the structures described in the publications, the structure described in Japanese Laid-open Patent Application No. 2-286468 is low in heat resistance, because coupling support between an outer shaft and an inner shaft is based only on a synthetic resin. The structure described in Japanese Utility Model Publication No. 58-51096 is arranged so that two steel balls are in contact with inner surfaces of corresponding roots in female serration grooves and that a relative displacement is permitted between the outer shaft and the inner shaft with plastic deformation at the two positions upon collision. Thus, such structure is likely to need a large force (a so-called collapse load) for decreasing the total length of the shaft for the collapsible steering apparatus. An increase in the collapse load could cause a failure to absorb a backward motion of the steering gear due to a collision that pushes the steering wheel 1 backward or changes the posture of steering shaft 2, and when the body of driver hits the steering wheel, a forward impact on the steering wheel cannot be absorbed, which exerts a strong impact on the driver's body colliding with the steering wheel and which then tends to cause serious injury to the driver. Therefore, this is not preferable.
Japanese Utility Model Application No. 4-18149 (Japanese Laid-open Utility Model Application No. 5-35542) describes the structures as shown in FIGS. 6 to 8 as arrangements for solving such a problem. FIGS. 6 and 7 show a first example of structure described in the application. This shaft 10 for collapsible steering apparatus is arranged so that the total length is decreased with application of an impact force in the axial direction because the outer shaft 11 with the inner shaft 12 are arranged so as to be capable of being displaced relative to each other in the axial direction. The outer shaft 11 is a circular cylinder throughout its length, where female serration grooves 13 are formed on an inner surface at one end of the shaft 11. The inner shaft 12 is a circular rod throughout its length, where male serration grooves 14 to be engaged with the above female serration grooves 13 are formed on an outer surface at one end of the shaft 12.
A recess 16 is formed at a portion corresponding to a root 15 in the male serration grooves 14 on the outer surface at one end of the inner shaft 12, and a steel ball 17, which is a hard member, is set in the recess 16. Then the female serration grooves 13 are engaged with the male serration grooves 14 so as to couple the outer shaft 11 with the inner shaft 12 and to cause a part of the steel ball 17 to intrude into a top part 18 of a thread in the female serration grooves 13. Namely, in the case of the shaft 10 shown in FIGS. 6 and 7, the outer shaft 11 and inner shaft 12 are coupled with each other by intruding a part of the steel ball 17 set in the recess 16 formed on the outer surface at the one end of inner shaft 12 into the top part 18 of the thread in the female serration grooves 13 formed on the inner surface of outer shaft 11. As described, because the coupling between the outer shaft 11 and the inner shaft 12 is based on the steel ball 17, the heat resistance of the coupling portion is sufficiently high as compared with the aforementioned structure with the coupling support only of the synthetic resin, thereby avoiding an event that the supporting force of the coupling portion becomes insufficient depending upon operation conditions.
In the case where, during primary collision of a car with another car or other object, the steering gear is pushed backward (in the case of use as the intermediate sheet 7) or in the case where, during secondary collision of the driver's body with the steering wheel (in the case of use as the steering shaft 2), a strong force is exerted on the shaft 10 in the axial direction, the steel ball 17 plastically deforms the top part 18 of the thread in the female serration grooves 13 to permit a relative displacement between the outer shaft 11 and the inner shaft 12, thereby decreasing the total length of the shaft 10. In the case of this shaft 10, because the steel ball 17 is in contact with the female serration grooves 13 at the top part 18 of the thread in the female serration grooves 13, a relatively small force is enough to cause the plastic deformation. Therefore, when this shaft 10 is used as the steering shaft 2 or as the intermediate shaft 7 (FIG. 5), the arrangement can prevent the collapse load necessary for decreasing the total length of the shafts 2, 7 from becoming large or can enhance the effect of preventing a great impact force from being exerted on the driver's body when colliding with the steering wheel upon a colliding accident.
In the structural example shown in FIGS. 6 and 7, a combination of the recess 16 and steel ball 17 are provided at only one point in the axial direction, but plural combinations of those may be provided at a plurality of portions for arrangements with a long serration engagement portion. The application also describes a second example shown in FIG. 8 wherein a plurality of recesses 16, 16 and steel balls 17, 17 are provided at plural portions in the circumferential direction. By such an arrangement wherein the recesses 16, 16 and steel balls 17, 17 are provided at the plural portions in the circumferential direction, a support force acting between the inner surface of outer shaft 11 and the outer surface of inner shaft 12 becomes substantially uniform throughout the circumferential direction, so that displacement of the shafts 11, 12 relative to each other may be effected more smoothly upon collision.
The prior art structures as shown in FIGS. 6 to 8, however, still have a point to be improved as described below. In the shafts 10 shown in FIGS. 6 to 8, rattle is stopped between the outer shaft 11 and the inner shaft 12 by intruding the steel ball(s) 17 held in the recess(es) 16 on the outer surface of inner shaft 11 into the top part(s) 18 of the thread(s) in the female spline grooves 13 formed on the inner surface of outer shaft 11. Thus, a portion of the top part 18, which is a portion strongly pressed by the steel ball 17 on the inner surface, is plastically deformed to have an increased width at the top part. Then a great frictional force acts on the engaging portion when the thus plastically deformed portion is engaged with the root 15 in the male serration grooves 14 formed on the outer surface of the inner shaft 12. This increases the stroke over which a large load is necessary for decreasing the total length of the shaft 10, which increases the possibility that a great impact is applied to the driver's body when colliding with the steering wheel upon collision.
In the original design of the shaft 10, when the total length of the shaft 10 is decreased by a length L.sub.1 shown in FIG. 6, which is a distance between the setting portion of the steel ball 17 and one end of the female serration grooves 13, and when the steel ball 17 has left the female serration grooves 13, a load necessary for further decreasing the total length of shaft 10 becomes extremely small. In other words, the original design is such that some load is necessary for the first stroke L.sub.1 to decrease the total length of shaft 10 upon a secondary collision, but a further decrease can be effected with a very small load over the stroke L.sub.2.
In contrast, in the case of actual shaft 10, the load necessary for decreasing the total length of the shaft 10 is great before the portion of the outer shaft 11 plastically deformed by the steel ball 17 leaves the male spline grooves 14 on the outer surface of inner shaft 12. Although the plastically deformed portion is normally a portion abutting the steel ball 17, the area of the plastically deformed portion increases along the stroke L.sub.1 as the total length of shaft 10 is decreased upon collision. Then, the load necessary for until the total length of the shaft 10 stays large until the entire plastically deformed portion in the range of L.sub.1 leaves the female spline grooves 13.
Accordingly, the length L.sub.2 in FIG. 6, which is a distance between the setting portion of steel ball 17 and the other end of female serrations 13, is added to the stroke L.sub.1 to define the range necessitating the great load for decreasing the total length of shaft 10. For example, according to measurements by the inventors, the axial load changed as shown in FIG. 9 when a strong force was applied in the axial direction of shaft 10 in order to decrease the total length of shaft 10. In FIG. 9, the vertical axis represents the amplitude of load and the horizontal axis represents an amount of decrease of shaft 10. The length L.sub.3 necessitating the great load for decreasing the total length of shaft 10 became approximately equal to L.sub.1 +L.sub.2 (L.sub.3 .congruent.L.sub.1 +L.sub.2).
If the load necessary for decreasing the total length of shaft 10 is kept large over the long stroke range (L.sub.3) or during a long time period as described above, a great impact is applied on the driver's body when colliding with the steering wheel upon collision, which is not preferable with regard to protecting the driver. The aforementioned application describes a technique of once extending and contracting the shaft 10 before the shaft 10 is incorporated inside the steering column 3 in order to stabilize the load necessary for decreasing the total length of shaft 10 upon collision. This technique can expect some improvement, but it cannot be said as a sure solution, because the loads exerted in production are often different in direction or magnitude from those exerted upon collision and therefore the plastic deformation by the steel ball 17 sometimes differs between them.